X-ray phase imaging is a method for detecting phase changes of X-rays by a sample and obtaining an image of the sample on the basis of the detection result.
As a method of the X-ray phase imaging, PTL 1 proposes a method for obtaining information related to a phase change of X-rays by detecting the amount of refraction of X-rays by a sample using a fact that X-rays are refracted by a phase change of the X-rays.
The principle of X-ray phase imaging method will be briefly described. In this method, first, X-rays are spatially divided by a grating which includes opaque objects (shield portions) that shield the X-rays and transparent objects (transmission portions) that transmit the X-rays. The divided X-rays become a discrete X-ray beam, the discrete X-ray beams are incident on a sample, and a discrete X-ray beams that are transmitted through the sample are detected by an X-ray detector. Thereby, how much a position of the discrete X-ray beam formed on the X-ray detector is shifted by the sample is known, and the amount of refraction of the X-rays can be obtained from the amount of the shift of the position (hereinafter referred to as “position shift amount”). Hereinafter, unless otherwise described, the position shift amount of the discrete X-ray beam in this description means the amount of positional shift of the discrete X-ray beam on the detector.
When the X-ray phase imaging is performed by the above-described method, generally, the smaller the width of the discrete X-ray beam to be used is, the higher the sensitivity of X-ray phase detection is.
The reason of this will be briefly described. The smaller the width of the discrete X-ray beam to be injected into the detector is, the smaller the intensity of the X-rays detected by each pixel of the detector is. On the other hand, the amount of refraction of X-rays generated by a certain sample does not depend on the width of the discrete X-ray beam, so that the position shift amount of the discrete X-ray beam does not depend on the width of the discrete X-ray beam. Thereby, the smaller the width of the discrete X-ray beam is, the larger the X-ray intensity change which is generated by the positional shift of the discrete X-ray beam and detected by each pixel with respect to the X-ray intensity detected by each pixel of the detector (that is, the X-ray intensity change which is generated by the presence or absence of the sample and detected by each pixel) is. Generally, the larger the X-ray intensity detected by the detector is, the larger the magnitude of the noise is, so that the larger the X-ray intensity change which is generated by the positional shift amount of the discrete X-ray beam and detected by each pixel with respect to the X-ray intensity, the smaller the probability that the X-ray intensity change is buried in the noise. As a result, the phase detection sensitivity of the X-ray imaging apparatus improves.
To reduce the width of the discrete X-ray beam, the width of the transparent objects of the grating is reduced. However, generally, it is difficult to manufacture a grating in which the width of the transparent objects is small. PTL 1 describes an X-ray imaging apparatus which includes two masks and in which the aperture size of masks can be adjusted by adjusting relative positions of the two masks. When the X-ray imaging apparatus is used, it is possible to obtain a discrete X-ray beam having a smaller width by using grating that is conventionally used.